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Review asked for this directly: the hardware layout above port_l2mc_set() would be better as documentation than as a comment, keeping only the two lines that say what the function does. doc/l2.md gains a section on static multicast entries, why delivery uses the forward action rather than the trap, and the SMI layout of the entry. doc/CpuPort.md gains the layout of the tag's flags and pmask words, with the byte order trap that cost an afternoon: writing the flags constant raw instead of through HTONS puts 0x0020 on the wire as 0x2000, which is EFID rather than LEARN_DIS, and the ASIC then leaves the 0x8899 header on the frame. The comments those paragraphs came from are replaced by a pointer to the file that now holds them.
3.9 KiB
The CPU Port
The RTL827x provide a CPU Port for a NIC on the 8051 side of the SoC.
Receiving packets
In order to receive packets on the ASIC side, bit 0 of RTL837X_REG_RX_CTRL (0x785c) must be set. Further bits in the register enable reception of various kinds of Ethernet frames. They should all be set in order for the firmware to decide what to do with them. To drop packets with incorrect Ethernet frame CRC already by the ASIC, clear bit 2 of this register.
Packets are received by either polling the RTL837X_REG_RX_AVAIL register (0x7874), which will be > 0 if data is within a ring-buffer on the ASIC side of the SoC. Alternatively, an interrupt can be triggered (EX1).
Data is transferred to the 8051 side by calling an SFR function. First, the frame header of the received frame will be copied over. For this, provide the destination memory location in xdata memory in SFRs B3 and B4 (little endian), the source location on the ASIC-side in SFRs B5/B6 (also little endian, found in RTL837X_REG_RX_RINGPTR, 0x787c) and execute the function by setting SFR_NIC_CTRL (B7) to the length to be transferred divided by 8, i.e. 1.
The frame header has the following format:
SS xx xx xP LL LH xx xx
SS: 8-bit sequence number
P: Port number
LHLL: Length of Ethernet frame (little endian)
xx: Unknown
Next, transfer the actual packet over by repeating the SFR function with a pointer to the frame on the ASIC directly after the frame header and a length as given by the length in the frame header + 7, again divided by 8.
The received frame will have an RTL proprietary Ethernet frame type of 0x8899 (RRPC) where normally the frame type 0x0800 for IPv4 would be located. Further 6 bytes follow describing the frame, before the normal IPv4 data starts. A documentation can be found here: TAG8899_COMMIT
After copying over header and frame, the frame is marked read in the ring buffer on the ASIC side by writing 0x1 to RTL837X_REG_RX_DONE (0x784c).
Transmissing packets
Packets are transmitted by preparing a frame-header plus frame in xdata memory and transferring both to the ASIC side via the SFRs. The ASIC will transmit packets if bit 0 of RTL837X_REG_TX_CTRL (0x7860) is set.
SS 07 00 00 LL LH 00 00
SS: 8-bit sequence number
07: Enables header and TCP checksum offloading to ASIC
LHLL: Length of the Ethernet frame
The Ethernet frame data starts immediately after the frame header in xdata memory. The frame is transferred to the ASIC side by setting SFRs B3 and B4 to the xdata source address of the frame header, and the ring pointer to the free space indicated by register 0x7890 multiplied by 8 and the MSB set. The length is given by the length of the frame plus 15, divided by 8.
Writing 0x1 to register 0x7850 will transmit the frame. The Ethernet frame checksum and the TCP checksum are automatically calculated (offloaded) by the ASIC before transmitting on the wire.
The RTL tag words
The frame header uses the Realtek Remote Control Protocol (RRCP) format or the like.
The flags word:
bit15 EFID_EN | 14:12 EFID | 11 PRI_EN | 10:8 PRI |
bit7 KEEP | 6 VSEL | 5 LEARN_DIS | 4:0 VIDX
All fields are in network byte order.
EFID_EN,EFID: look the destination up under this filtering ID instead of the port's ownPRI_EN,PRI: force the given priority on the frameKEEP: keep the 802.1Q tagging of the frame exactly as injected, bypassing the egress tagging rules of the portVSEL,VIDX: classify the frame into the VLAN at this index of the VLAN tableLEARN_DIS: do not learn the source address from this frame
The pmask word: bit 15 is ALLOW, bits 14 to 0 are a port mask.
ALLOWclear: the mask is the egress set, the frame goes to exactly the ports givenALLOWset: the ASIC looks the destination up as usual and the mask only limits which ports the result may use